Pharmaceutics, Dosage Form Design, and Compounding
These questions test design decisions - which dosage form, which excipient, which release pattern - and the arithmetic that turns a formula into a dispensed product.
Design logic
- Biopharmaceutics classification: class I and III are straightforward; class II needs particle-size reduction or a lipid system; class IV is hardest.
- Excipients are chosen by function: diluent, binder, disintegrant, lubricant, coating, antioxidant, preservative, buffer.
- Sterile products need validated sterilisation (autoclave 121 °C), pyrogen control and attention to tonicity (serum ≈ 290 mOsm/kg; 0.9% NaCl is the usual reference).
- Stability dictates storage and expiry; suspensions add sedimentation, caking and dose-uniformity risk.
Worked example
Compound 500 g of 2% w/w hydrocortisone cream from 1% and 5% creams. Alligation: parts of 1% = 5 − 2 = 3; parts of 5% = 2 − 1 = 1, so the ratio is 3:1. 1% cream = 500 × 3/4 = 375 g; 5% cream = 500 × 1/4 = 125 g. Check: (375 × 0.01) + (125 × 0.05) = 3.75 + 6.25 = 10 g, and 10/500 = 2% ✓.
Traps
Mixing w/w with w/v (1% w/v is 1 g in 100 mL, not 100 g); assuming volumes are additive; ignoring the displacement value when a drug is incorporated into a suppository base; assigning a compounded or repackaged product the bulk product's expiry date; using tap water where purified water is specified.